There is a continuing need for improved chemical sensing devices to be able to continuously and reliably measure a particular analyte or analytes in a process stream. Since the introduction of solid and gas lasers more than two decades ago, there has been interest in a class of optoelectronic devices that rely upon the use of a light beam in solids for their operation for chemical sensing. An example of a solid is an optical fiber. Typical devices falling within the scope of that class are optical modulators, frequency mixers, parametric oscillators, and the like. More recently, there has been interest in using integrated optics with the application of thin-film technology to optical circuits and devices. However, these devices are not generally used for chemical sensing.
Multiple internal reflectance has been used as a means to produce an evanescent wave that can interact with an analyte or analytes in the reaction volume adjacent to a light-propagating element. The interaction of the analyte or analytes modulates the evanescent wave. This is a means for analyzing the presence and concentration of a particular analyte or analytes in the reaction volume. The light-propagating elements or crystals have been commercially available as large, self-supporting cylindrical rods, rectangular crystals, or prisms and sold as accessories for laboratory spectroscopic instruments.
These thick elements were generally designed for the fundamental vibrations of the substance that usually occur in the infrared. Thus the thickness of the element was designed to produce from about 1 to 10 bounces of light (assuming a ray-type propagation of electromagnetic radiation rather than by waveform) at the upper surface of the element. This low number of interactions (i.e., bounces) of light with the analyte or analytes in the reaction volume diminishes the sensitivity of the element to adequately measure the very intense fundamental vibrations. Thus, thicker, self-supporting elements may not have the sensitivity to analyze the weaker absorbing analytes, especially in the near-infrared spectral region where vibrational overtones and combination modes occur. The near-infrared region is an important spectral region because many optical fibers are available to transmit light energy in this spectral region and detector arrays are available for rapid analysis.
Thus there is a need in the art to make the elements thinner, thereby increasing the interaction of the propagating wave of electromagnetic radiation with the analyte or analytes in the reaction volume. Another problem with thinner elements is that they no longer are self-supporting, which diminishes or renders more difficult the ability to couple light into the elements. Thus there is a need in the art to be able to efficiently couple light into extremely thin elements, such as waveguides.
Planar waveguide technology began to be actively pursued in the 1960s in the semiconductor industry, with the goal of producing integrated optical circuits for microwave devices and networks. Much of the work for thin-film, planar waveguides and methods to couple light into them were also directed to these semiconductor industry objectives.
Much research has been directed toward the development of integrated optical components for semiconductor applications employing optical circuits. These devices typically employ thin-film dielectric or polymeric waveguides that are often less than 1 .mu.m thick. This technology has advanced due to progress in the areas of photolithography, thin-film processing, and miniaturized laser light sources.
Optical propagation through a planar waveguide is described according to the following equations: ##EQU1## wherein t is the film thickness; n.sub.i is the refractive index of the ith layer; and the subscripts 1, 2, and 3 refer to the sample, waveguide, and substrate, respectively. X is the angle in degrees between the axis of propagation of the optical mode (m) and the waveguide normal. Values of X can range between the critical angle (sin.sup.-1 n.sub.3 /n.sub.2) to near 90.degree.. A large value of X represents a mode that is traveling nearly parallel to the surface of the waveguide. TE represents a polarization in which the electric fields are perpendicular to the plane of incidence spanned by the wave normal and the normal to the interface. TM modes represent a polarization in which the magnetic fields are perpendicular to the plane of incidence.
A variety of thin films were examined for use as waveguide materials. Common waveguide materials include glasses; oxides, such as tantalum pentoxide; nitrides, such as silicon nitrate; and polymers, such as polystyrene and polycarbonate. A thin-film waveguide is characterized by a thin film with a higher refractive index than the materials (liquid or solid or gas) that bound its upper and lower surfaces. As a result, light or electromagnetic radiation can be focused through the materials surrounding the waveguide in a way that will cause light to be coupled into and propagated through the waveguide. What is required is a means to match the propagation constant for allowed waveguide propagation modes in order to couple externally generated light into the waveguide. Prism couplers had been used to accomplish this procedure.
The use of prisms to couple electromagnetic radiation into the waveguide has a number of disadvantages. The first disadvantage is the incompatibility of a prism structure with the overall planar geometry of a planar, thin-film waveguide. Second, the prisms must have a higher refractive index than the waveguide, which already may have a high refractive index (greater than 2.0). This limits the choice of prism materials. A third disadvantage is the need to maintain the coupling condition (have a constant space) between the prism and the waveguide material. Without the use of a bonding material, the prism often rests on small dust particles as it is clamped onto the waveguide. This makes it difficult to reproduce optical readings. Moreover, as a chemical sensor, the wicking of liquids or vapors into the volume between the prism and the waveguide poses problems for the performance of the optical sensor device by affecting the ability to reproducibly couple light into or out of the thin-film waveguide.
Some investigators have attempted to solve the third problem by attaching flow cells to a region on the waveguide and between the prisms and by not moving the prisms once they are clamped in place. See Ives et al., Appl. Spect. 68-72, 1988 and 41:636, 1988. However, this results in induced losses for the higher order modes passing under the gasket and causes cross-coupling between modes in multiple-moded waveguides. This may be adequate for research or laboratory applications but is not useful for field or commercial applications. Other approaches to solve the problem of light coupling into waveguides have used tapered ends to the waveguides or end couplers to "end fire" electromagnetic radiation in order to propagate through the waveguide.
Yet another approach has been the use of a grating on the upper surface of a thin-film, planar waveguide as a surface relief grating. Grating fabrication typically involves spin-casting a thin layer of a polymeric photoresist material, exposing the photoresist material to a desired pattern, and developing the exposed photoresist to leave the pattern on the waveguide film (Dakss et al., App. Phy. Lett. 16:523, 1970).
One problem has been that solvents and other chemicals used to fabricate gratings on polymeric waveguides may adversely affect the waveguide itself. Any chemical reaction with the grating material complicates signal analysis. Gratings have also been produced in the surface layer of inorganic waveguide materials by embossing the grating pattern in dip-coated gel films made from organometallic films before firing. This is referred to as a "surface relief grating." (See Lukosz et al., Opt. Lett. 8:537, 1983).
Moshrezadeh et al., Appl. Opt. 26:2501, 1987, refers to the use of a "buried" grating below the waveguide as an "incoupling grating" to characterize the nonlinear optical properties of the thin polymeric film.
U.S. Pat. No. 4,815,843 refers to the use of light fired into the waveguide from the substrate side, with buried gratings between the substrate and the waveguide. U.S. Pat. No. 4,815,843 refers to changes in measured signal due to changes in the refractive index induced by a chemisorbed layer formed over the grating from the reaction volume. Measurements are made by a change in angle with a rigid oxide waveguide between a conochromatic light source and an input grating at a fixed angle detector. This reference does not couple light into the waveguide from the substrate side. Other papers by Tiefenthaler refer to the same adsorptive effect using surface relief, embossed gratings rather than buried gratings.
Accordingly, there is a need in the art to design a planar, thin-film spectroscopic sensor that emphasizes sample spectral absorption rather than having the electromagnetic radiation coupling affected by adsorption from the reaction volume and can couple electromagnetic radiation into a thin-film waveguide without end-firing into too thick a material.